1. Field of the Invention
The present disclosure relates to an image forming apparatus.
2. Description of the Related Art
In a conventional general image forming apparatus, presence/absence of a sheet (for example, a paper) is detected by a transmission-type sensor and its light shielding flag. In this arrangement, presence/absence of sheet is detected when the transmission-type sensor respectively switches between light transmitting/light shielding with an operation of the light shielding flag. In this method, however, it is likely that, depending on a size of the light shielding flag, the sheet cannot correctly be detected in a case where an edge of sheet is not in contact with the sheet tray due to, for example, the sheet placed on the sheet tray which is folded or curled.
Then, at the portion of the sheet tray where occurrence of such problem is concerned, reflection-type sensor (for example, long distance type reflection-type sensor, blank sheet detection distance of which is more than or equal to 20 [mm]) is used to allow detection of the sheet even the “paper curl” is caused. The sensor of this type irradiates light on a target from light emitting element to detect reflected light. The presence/absence or size of the sheet is detected in this manner so that distance which is detectable depends on light amount of the light emitting element. In general, light emitting diode (LED) used as the light emitting element has a feature that the longer accumulated lighting time of the LED gets, the less amount the LED emit lights. Further, as compared to the transmission-type sensor, the long distance type sensor emits a larger amount of light. Thereby, due to aged deterioration, the light amount of the LED rapidly reduces. Thus, to suppress deterioration of the LED of the reflection-type sensor, it is necessary to minimize the lighting time of the LED.
For example, Japanese Patent Application Laid-open No. 2003-182886 discloses a device in which the reflection-type sensor as mentioned is employed as a sensor on an original tray of an automatic document feeder. Also, in the device, the light emitting element is turned on just before feeding operation is started to detect length of the original and the light emitting element is turned off after the detection.
Besides the automatic document feeder, a portion at which wrong detection, caused by the sheet curl, is concerned in the image forming apparatus includes a manual feed tray. In the following, the difference of sheet size detection mechanism between the automatic document feeder and the manual feed tray is described.
In case of the automatic document feeder, it is less likely that the sheet is intentionally left. The original is placed on the original tray just before reading operation of the original is started. When the reading of the original is finished, no original is left on the original tray. That is, it is less likely that the original is left on the original tray for a long time. On the other hand, in case of the manual feed tray, a special sheet, which cannot be fed from a sheet feeding cassette, sometimes remains set intentionally on the manual feed tray. The special sheet includes postcard, envelope, etc. Due to this, it is highly likely that the sheet is left on the manual feed tray for a long time. For example, it is assumed that the image forming apparatus is switchable between a normal power mode and a power saving mode. In the normal power mode, normal power is supplied for performing image formation. In the power saving mode, less power is consumed as compared to the power consumed in the normal power mode. In a case where the sheet is already set on the manual feed tray when returning from the power saving mode, sheet size is detected immediately after the return by a sheet size sensor having a LED. Thereby, every time the image forming apparatus returns from the power saving mode, the LED of the sheet size sensor is turned on. This results in rapid progression of deterioration of the LED of the sheet size sensor so that the detection distance is reduced, which increases the possibility that the sheet size is wrongly detected, which is a problem. The main object of the present disclosure is to provide an image forming apparatus which can control the turning-on of the light emitting element according to a state of the sheet on the manual feed tray.